1. Field of the Invention
The present invention relates to a code reuse apparatus in a code division multiple access (CDMA) wireless communication system using beamforming by an antenna array, and a code reuse method therefor.
2. Description of the Related Art
For efficient use of limited frequency resources, a code division multiple access (CDMA) scheme using an identical frequency bandwidth divided by codes, and a space division multiple access (SDMA) scheme using an antenna array, are being utilized. The standardization organization has recently discussed methods for combining the CDMA scheme with the SDMA scheme in order to enhance frequency efficiency.
In general, the SDMA scheme may be operated by beamforming using an antenna array. Here, if the beamforming is applied to a downlink, unlike an uplink, it is difficult to accurately find space information of a user. Accordingly, the SDMA scheme considers gains obtained by power reduction of a base station rather than code reuse.
As a communication system using an antenna array, a fixed beamforming method suggested by Nokia, an eigenbeamforming method suggested by Siemens, and an enhanced eigenbeamforming method suggested by Samsung are presently being discussed by the 3GPP standardization association. The subject of reusing codes in a beamforming system was proposed by Nokia, which has claimed that code reuse is possible as Nokia uses the fixed beamforming method in which a beam shape is pre-fixed.
A conventional code allocation method in a CDMA system will now be briefly described.
FIG. 1A illustrates a diagram depicting a conventional code allocation method in a CDMA system. A mobile communication system shown in FIG. 1A is constructed of a base station 140, a first mobile station 110, a second mobile station 120, and a third mobile station 130. The base station 140 has three antennas 141,142, and 143, each of which manages a single sector spanning 120°, and is shielded by shielding means 144.
For example, the antenna 141 transmits the same code to the first, second, and third mobile stations 110, 120 and 130, which are positioned in the managed sector. Each of the mobile stations 110, 120 and 130 communicates by recognizing only the code known to that mobile station among the codes transmitted from the base station. That is, the first mobile station 110 recognizes only the code known to the first mobile station 110, and the second mobile station 120 recognizes only the code known to the second mobile station 120, etc. In this method, the mobile stations communicate with the base station.
In the conventional system shown in FIG. 1A, even though each mobile station communicates only with the code it knows, interference caused by transmitting the same codes to all the mobile stations cannot be avoided.
FIG. 1B illustrates a conceptual diagram of a code allocation method used in an eigenbeamforming method.
A mobile communication system shown in FIG. 1B is constructed of a base station 180, a first mobile station 150, a second mobile station 160, and a third mobile station 170. The base station 180 includes antenna arrays 181, 182, and 183. The antenna array 181 communicates with the mobile stations positioned in a sector A. Here, the antenna array 181 does not transmit all the codes to each of the mobile stations. Rather, the antenna array 181 generates and transmits a beam to each mobile station using a code allocated to each mobile station. The first mobile station 150 receives only a first beam 151 transmitted from the antenna array 181, the second mobile station 160 receives only a second beam 161 transmitted from the antenna array 181 and the third mobile station 170 receives only a third beam 171 transmitted from the antenna array 181. Therefore, although there is a slightly overlapped sector (a), the first and second mobile stations 150 and 160 may communicate with the antenna array 181 without co-interference.
A weight value used for beamforming is generated by optimally combining long-term information including space-axis information and short-term information including time-axis information. The space-axis information reflects a departure of angle (DOA), which is an angle formed by the base station with the mobile station, as well as an angle spread (AS). The time-axis information reflects Doppler shifting and multipath. The long-term information, which is affected by the location of the mobile station and reflects a long-term variation of a channel, varies very slowly. On the other hand, the short-term information is affected by the movement of the mobile station and reflects only an instantaneous variation of a channel.
In the mobile communication system shown in FIG. 1B, in a case that a number of mobile stations equal to the number of codes usable by the base station are already in communication with the base station, when a new mobile station requests communication, there is no code left to be allocated to the new mobile station, even when the base station has plenty of power to spare. For example, suppose that the power of the base station is 100% and the number of codes usable by the antenna array 181 is 3. When the first mobile station 150 uses 10% of the power of the base station via the first code, the second mobile station 160 uses 20% of the power of the base station via the second code, and the third mobile station 170 uses 20% of the power of the base station via the third code, even though only 50% of the power of the base station is being used, there is no code left to be allocated to a new user that enters the sector A. Accordingly, the new user cannot communicate with the base station and power of the base station is wasted.